1. Field of the Invention
This invention relates to novel thermal dye transfer constructions, and in particular to dye donor elements. This invention further relates to donor elements based on mixtures of green 1,4-bis(arylamino) anthraquinones with blue 3-dicyanomethylidene-2,3-dihydothiophen- 1,1-dioxide derivatives.
A further aspect of this invention is the provision of dye donor elements which, when imaged, give rise to dye images of excellent light fastness, high density, low hue error, and low turbidity, such images being useful for a variety of applications including color proofing.
2. Background of the Art
The term thermal transfer printing covers two main technology areas. In thermal transfer printing of textiles, a donor sheet is coated with a pattern of one or more dyes, contacted with the fabric to be printed, and heat is uniformly administered, sometimes with concomitant application of a vacuum. The transfer process has been much studied, and it is generally accepted that the dyes are transferred by sublimation in the vapor phase. Pertinent references include: C. J. Bent et al., J. Soc. Dyers Colour., 85, 606 (1969); J. Griffiths and F. Jones, ibid., 93, 176, (1977); J. Aihara et al., Am. Dyest. Rep., 64, 46, (Feb., 1975); and C. E. Vellins in "The Chemistry of Synthetic Dyes", K. Venkataraman, ed., Vol. VIII, 191, Academic Press, New York, 1978.
The other area covered by the term thermal printing is thermal imaging, where heat is applied in an imagewise fashion to a donor sheet in contact with a suitable receptor sheet to form a colored image on the receptor. In one embodiment of thermal imaging, termed thermal mass transfer printing, as described for instance in U.S. Pat. No. 3,898,086, the donor is a colorant dispersed in a wax-containing coating. On the application of heat, a donor layer in the construction melts or is softened, and a portion of the colored donor coating transfers to the receptor. Despite problems with transparency, pigments are generally the colorants of choice to provide sufficient light fastness of the colored image on the receptor. Another embodiment is termed variously thermal dye transfer imaging or recording, or dye diffusion thermal transfer. In this embodiment, the donor sheet comprises a dye in a binder. On imagewise application of heat, the dye, but not the binder, is transferred to the receptor sheet. A recent review has described the transfer mechanism as a "melt state" diffusion process quite distinct from the sublimation attending textile printing. [See: P. Gregory, Chem. Brit., 25, 47 (1989)].
This same review emphasizes the great difficulty of finding or synthesizing dyes suitable for diffusive thermal transfer, stating that "It is significant that of the one million or so dyes available in the world, none were fully satisfactory". Among the failings of said dyes are inadequate light and heat fastness of the image and insufficient solubility of dyes for coating in the donor sheet. As has been noted previously, light fastness is also a problem in mass transfer imaging systems. In fact, achieving adequate light fastness is probably the single biggest challenge in these constructions. In large measure this is the result of the diffusive thermal transfer dye image being a surface coating a few microns thick. The dye is thus readily susceptible to photooxidative degradation. In contrast, textile fibers, which are 100 times thicker, are uniformly dyed throughout their depth, so that fading in the first few microns at the surface is of little practical importance. In consequence, it is common to find that dyes showing good light fastness in textile printing exhibit very poor photostability in diffusive thermal transfer imaging (see e.g., U.S. Pat. No. 4,808,568), and there remains a strong need for improved dyes for the latter application.
Although thermal printing of textiles bears a superficial resemblance to diffusive thermal dye imaging, they are in reality quite different processes with distinct properties and material requirements involved. Thermal printing occurs by a sublimation process, so that substantial vapor pressure is a prime criterion for dye selection. In diffusive dye imaging, high vapor pressure of the dye contributes to undesirable thermal fugacity of the image. For the melt state diffusion process involved in this situation, melting point is instead a better basis for dye selection. Diffusive dye transfer is a high resolution dry imaging process in which dye from a uniform donor sheet is transferred in an imagewise fashion by differential heating to a very smooth receptor, using heated areas typically of 0.0001 square inches or less. In contrast, the thermal printing of textiles is of comparatively low resolution, involving contemporaneous transfer by uniform heating of dye from a patterned, shaped or masked donor sheet over areas of tens of square feet. The typical receptors printed in this manner are woven or knitted fabrics and carpets. The distinct transfer mechanism allows such rough substrates to be used, while diffusive imaging, where receptors with a mean surface roughness of less than 10 microns are used, is unsuitable for these materials. Unlike diffusive thermal dye imaging, the transfer printing process is not always a dry process; some fabrics or dyes require pre-swelling of the receptor with a solvent or a steam post-treatment for dye fixation. Though the transfer temperatures for the two processes can be similar (180.degree. to 220.degree. C.), diffusive dye transfer generally operates at somewhat higher temperatures. However, in a manner strikingly reflective of the differences in mechanism involved, diffusive dye transfer involves times of around 5 msec, whereas thermal printing normally requires times of 15 to 60 sec. In accord with the sublimation process involved, thermal printing often benefits from reduced atmospheric pressure or from flow of heated gas through the donor sheet. Thermal printing is a technology developed for coloring of textiles and is used to apply uniformly colored areas of a predetermined pattern to rough substrates. In contradistinction, diffusive dye transfer is a technology intended for high quality imaging, typically from electronic sources. Here, a broad color gamut is built with multiple images from donors of the three primary colors onto a smooth receptor. The different transfer mechanism allows the requirement for grey scale capability to be fulfilled, since the amount of dye transferred is proportional to the heat energy applied. In thermal printing grey scale capability is expressly shunned, because sensitivity of transfer to temperature decreases process latitude and dyeing reproducibility.
It has now been found that mixtures of blue 3-dicyanomethylidene-2,3-dihydothiophen-1,1-dioxide derivatives and green 1,4-bis(arylamino) anthraquinones (the "Colour Index" identifies one of these derivatives, 1,4-bis(tolylamino) anthraquinone as Solvent Green 3) can be beneficially used in thermal dye transfer imaging. When these dye mixtures are used to prepare cyan dye donor constructions, the resultant transferred images exhibit improved light fastness and lower hue error and lower turbidity over comparable materials known in the art.
The thermal printing art, in teaching the use of production of full color images [Mitsubishi Kasei R & D Review, 3, (2), 71-80 (1989)], states that "in order to achieve a recorded good showing wide color reproduction range, it is necessary that the absorption spectral characteristics of the three primary color dyes be correct". It is noted that "each dye should absorb one third of the visible wavelength band while allowing the remaining 2/3 to be transmitted, and show high color purity, which does not allow overlapping of each absorption". Additionally, the prior art (e.g., U.S. Pat. No. 4,923,846) teaches that ". . . in heat transfer recording, if the color characteristics of the three colors of cyan, magenta, and yellow are not [low], the intermediate colors become turbid colors with low chroma, whereby no good color reproducibility can be obtained". In view of these disclosures, it is surprising that a green dye, such as a 1,4-bis(arylamino) anthraquinone, with absorption in the blue region of the visible spectrum, would be useful for preparation of cyan dye donor ribbons for dye diffusion transfer imaging with reduced hue error, and there is indeed very little mention of the use of green dyes in the dye diffusion transfer art.
Research Disclosure 32019 (Agfa-Gevaert, December 1990) lists ninety dyes, three of which are green 1,4-bis(arylamino) anthraquinones, which are used in dyeing fabrics and can also be used in thermal recording. No mention is made of their use as mixtures or their suitability for making cyan images.
Japanese Kokai 62-064,595 claims dyes for thermal printing that have inorganic/organic ratios less than or equal to 2.30, and lists 1,4-bis(4-methylphenylamino) anthraquinone, which is Solvent Green 3, as an example of such a dye. The same dye is listed as an example of a dye falling within both claims of Japanese Kokai 4-085,081 that specify a thermal donor sheet characterized by a diffusible dye having fluorescent character (emitting between 400-600 nm) and a binder comprising a whitening agent. Because of metamerism, fluorescence is undesirable in printing or recording applications, such as color proofing, where matching of colors is important. It is therefor unobvious that this anthraquinone dye could be used in a color proofing application. Solvent Green 3 is also disclosed in Japanese Kokai 61-268,495 which claims dyes for thermal printing having greater than or equal to five ring structures in the molecule and molecular weight greater than or equal to 350.
The 3-dicyanomethylidene-2,3-dihydothiophen-1,1-dioxide derivatives are well known in thermal transfer imaging, used alone and as mixtures. U.S. Pat. No. 5,036,041 lists several examples, each of which is used as a mixture with another dye whose wavelength of maximum absorption (.lambda.max) is 660 nm or higher.
U.S. Pat. No. 4,990,484 claims mixtures of indoanilines and another dye with .lambda.max between 560 and 700 and molecular weight greater than 345, and provides a 3-dicyanomethylidene-2,3-dihydothiophen-1,1-dioxide derivative as a component of the mixture in one example. U.S. Pat. No. 4,923,846 lists a dye of the same class as one preferable for making a cyan donor element, the image from which has specified hue error and turbidity.
A number of patents claim or disclose examples of 3-dicyanomethylidene-2,3-dihydothiophen-1,1-dioxide derivatives (these are blue dyes; Disperse Blue 354 is one example) used as mixtures with a 1-alkylamino-4-arylaminoanthraquinone derivatives (blue dyes), including Japanese Kokai 1-077,583 and 1-077,584 and U.S. Pat. Nos. 4,720,480 and 4,820,686. U.S. Pat. No. 5,077,264 claims the same mixture with an indoaniline dye included. Japanese Kokai 3-007,387 provides an example of this class of dye used to prepare a black dye diffusion transfer donor element.